Various processes for the production of alkylbenzene by the alkylation of benzene with an olefin are known in the art. Among the most common olefins used are ethylene and propylene. The alkylation of benzene with ethylene produces ethylbenzene. The alkylation of benzene with propylene produces cumene.
Alkylbenzenes, such as ethylbenzene and cumene (isopropylbenzene) are important industrial chemicals. In particular, ethylbenzene is commonly used to produce styrene, which may be polymerized to produce polystyrene, and cumene may be used as an additive for high-octane fuels or to produce phenol and acetone. Various methods are known for the production of alkylbenzenes, including ethylbenzene, which may be made by the alkylation of benzene with ethylene, as follows:C6H6+C2H4→C6H5C2H5
Successive alkylations generally occur, producing diethylbenzenes and other higher ethylated benzenes.
The following reaction is typical:C6H5C2H5+C2H4→C6H4(C2H5)2
Other coupling reactions occur to a minor extent, yielding materials such as butylbenzenes, diphenylethanes and higher boiling compounds. The mixture may be distilled to recover ethylbenzene, benzene and higher ethylated benzenes, and the higher ethylated benzenes may be transalkylated with benzene to form additional ethylbenzene. Processes related to alkylation reactions are disclosed in U.S. Pat. No. 5,003,119 to Sardina et al., which is incorporated by reference herein.
As an example, alkylation reactions may take place in a single fixed-bed reactor; and may occur adiabatically, i.e., no external heating or cooling is supplied, over a zeolite catalyst at an operating pressure high enough to maintain the reactor contents in the liquid phase. Carrying out the reaction in the liquid phase is more efficient than doing so in the gas phase and requires less catalyst. The reaction may be carried out with multiple catalyst beds in series, e.g., the benzene may be fed to the first catalyst bed and the ethylene may be fed separately to each catalyst bed. This multi-stage injection of ethylene may be used because it provides high local benzene-to-ethylene ratios in the catalyst beds for improved product selectivity, purity and extended catalyst run length. The reactor design parameters may be adjusted to ensure the optimum temperature profile for each catalyst bed, resulting in increased catalyst run times and minimum by-products.
While high purity ethylene is ideal for producing alkylaromatics, it is more expensive to produce than dilute ethylene. Dilute sources of ethylene are readily available, and alkylators may be configured in various ways to optimize alkylaromatic production with ethylene feeds of different concentrations. However, ethylene feeds with ethylene concentrations lower than about 70 mol % generally are not suitable to produce alkylaromatics because non-ethylene components in the feed do not dissolve in alkylators at reasonable pressures, which are typically less than 1500 psig. This results in the alkylator operating in the gas phase, which is less efficient and which consumes far more catalyst than a liquid phase alkylation reaction. Even ethylene feeds containing concentrations of ethylene higher than 70 mol % are not suitable for producing alkylaromatics if they also contain significant amounts of inert low molecular weight impurities (which do not react with the catalyst but which have relatively low boiling points), e.g., hydrogen, methane, ethane, nitrogen, carbon dioxide, carbon monoxide and, less commonly, butane and pentane. Hydrogen, methane, nitrogen, carbon dioxide and/or carbon monoxide, in amounts of about 5 mol % or higher, are especially problematic because the low boiling points of these low molecular weight impurities make them particularly difficult to dissolve in the alkylator.
Thus, there is a need for a process which allows for the efficient alkylation of aromatics, such as benzene, to form alkylaromatics, such as ethylbenzene. Further there is a need for a process which utilizes dilute sources of olefin, such as ethylene, minimizes olefin waste and reduces the amount of catalyst consumed during the alkylation reaction.